How to push to an array using index.of and if statement

I'm trying to figure out how to push an additional value to an array if my index.Of is equal or greater than 0. However the below code doesn't seem to work, I don't get any errors and I've checked the index.0f and it always equals 0.

var groups = ['a,b,c,']

function myfunction () {
    
    if (groups.indexOf('a,b,c,') >= 0) {
       groups.push('d') 
    }
}

console.log(groups);